Field of the Invention
The present invention relates to a refractive index distribution measuring method and a refractive index distribution measuring apparatus for measuring a refractive index distribution of an optical element.
Description of the Related Art
In a method for manufacturing a lens by molding, three physical quantities including the shape, the refractive index, and the refractive index distribution deviate from design values. In particular, defects in the refractive index distribution that occurs in the interior of the lens adversely affect optical performance. Therefore, in manufacturing a lens by molding, a technique for nondestructively measuring the refractive index distribution of the molded lens under the condition that the shape and the refractive index differ from the design values is required.
In a measuring method disclosed in Japanese Patent Application Laid-Open No. 2014-196966, a measurement cell in which a test object is sandwiched by a first reference element and a second reference element and in which a gap between the test object and the first reference element and a gap between the test object and the second reference element are filled with matching oil is formed. The refractive indices of the first and second reference elements are assumed to be substantially the same as the refractive index of the test object. Herein “substantially the same” means that values are within a negligible difference. Therefore, for the refractive indices of the first and second reference elements to be substantially the same as that of the test object, differences in optical performance of the test object would be negligible as compared to the optical performance of the first and second reference elements. The surface shapes of the first and second reference elements are the reverse of the surface shape of the test object. Then, interference fringes of the measurement cell are measured, phase distribution data is calculated from the interference fringes, and the difference between the phase distribution data and previously set reference data is calculated, to determine the refractive index distribution of the test object.
In the measuring method disclosed in Japanese Patent Application Laid-Open No. 2014-196966, by using the matching oil and the reference elements whose refractive indices are assumed to be substantially the same as the refractive index of the test object, the influences of refraction by the test object are cancelled to determine the refractive index distribution of the test object. However, in the method for manufacturing a lens by molding, differences occur among the refractive index of the test object, the refractive indices of the reference elements, and the refractive index of the matching oil because not only the shape of the test object but also the refractive index of the test object deviates from the design value for different molding conditions.
Due to the differences in the refractive indices, the influences of refraction by the test object cannot be completely cancelled. Therefore, an error in the shape of the test object (deviation from the design value of the shape) occurs as an error in the refractive index distribution. Further, just the difference between the refractive index of the test object and the refractive indices of the reference elements also causes an error in the calculation of the refractive index distribution. Therefore, in order to determine the refractive index distribution with high precision, a computation for correcting the influences of the error in the shape of the test object and the error in the refractive index of the test object (deviation from the design value of the refractive index) is required.
The present invention provides, for example, a refractive index distribution measuring method and a refractive index distribution measuring apparatus for allowing the refractive index distribution of a lens to be nondestructively measured with high precision.